was encoding videos for web as mp4 and webm, some of the mp4 converted from dvgrab were having problems playing...eventually came up pix_fmt were yuv422.

ffmpeg -i in.dv -vcodec libx264 -pix_fmt yuv420 -s 480x360 -vb 500K -acodec aac -strict experimental -ac 2 -ar 44100 -ab 100K -thread 0 -maxrate 600K -bufsize 600K temp.mp4
qt-faststart temp.mp4 out.mp4
rm temp.mp4

ffmpeg -i in.dv -vcodec libvpx -pix_fmt yuv420 -s 480x360 -vb 672K -acodec libvorbis -ac 2 -ar 44100 -ab 100K -thread 0 -maxrate 772K -bufsize 772K out.webm
...Think thats the does it.